1. Field of the Invention
This invention generally relates to a source for emitting a laser beam, and, in particular, to an apparatus for emitting a laser beam using a semiconductor laser. More specifically, the present invention relates to a light source assembly using a semiconductor laser, which is particularly suitable for use in a light scanning apparatus, such as a laser printer.
2. Description of the Prior Art
A laser printer is well known in the art, and it often uses a semiconductor laser as its laser beam source. Since a laser beam emitted from such a semiconductor laser is divergent in nature, use is also made of a collimating lens to make the laser beam emitted from the semiconductor laser collimated before being used. In some cases, the semiconductor laser and the associated collimating lens are provided in a unit structure to define a laser beam emitting assembly. In this case, the laser beam emitted from the assembly is collimated and thus it can be used in an imaging operation. Thus, the collimated laser beam emitted from the beam source assembly is scanned by scanning means and focused onto an imaging surface by focusing means to form an electrostatic latent image on the imaging surface.
On the other hand, it is also well known that the intensity of the laser beam emitted from a semiconductor laser fluctuates appreciably depending on the temperature of the surrounding atmosphere. In the case where the semiconductor laser is used as a light source in a light scanning system, such as a laser printer, it is particularly required that the intensity of the laser beam be substantially at constant. In order to keep the laser beam intensity at constant, provision is normally made of an optically coupled feedback loop for stabilizing the laser beam intensity at constant. Such an optically coupled feedback loop normally includes a photodetector for receiving the laser beam emitted from the semiconductor laser and a controlling device for controlling the level of the driving current to be supplied to the semiconductor laser so as to keep the intensity of the emitted laser beam to be at constant.
However, since there is a scatter in the sensitivity and the angle of divergence of an emitted laser beam from one semiconductor device from another and there is also a scatter in the sensitivity of the monitoring photodiode, there is a scatter in the relation between the laser beam intensity of the semiconductor laser and the output current of the photodiode. Thus, in the case where the laser beam emitting assembly including the semiconductor laser beam and the photodiode is to be replaced with another assembly for some reason, a power meter must be placed on a photosensitive member for measuring the actual power of the laser beam emitted from the semiconductor laser and a separate gain adjusting unit including, for example, a variable resistor is used to adjust the level of the intensity of the laser beam emitted from the semiconductor laser. This is extremely disadvantageous because the adjustment must be carried out after installment of a semiconductor laser or a laser beam emitting assembly including a semiconductor laser in an apparatus, such as a laser printer. In addition, a power meter must be used for carrying out the adjustment. The characteristic of the power meter also fluctuates, and, thus, the above-described prior art approach does not produce satisfactory results at all times.